The present invention related to the field of optical waveguides in general and in particular to a method and apparatus for reducing dispersion in optical waveguides.
Single mode optical waveguides (or optical fibers) are limited in bandwidth by dispersion. A pulse of given shape introduced into the input end of an optical waveguide will be spread as the pulse is transmitted along the length of the optical waveguide because of dispersion, the variation of wave phase velocity with wave frequency which is an inherent property of the materials comprising the guide as well as of guided modes themselves. For a particular waveguide with a given set of optical properties, the amount by which a pulse spreads is proportional to the length of the optical waveguide. Pulse spreading cannot be permitted to proceed to the point where adjacent optical pulses overlap. Thus, for a given pulse shape and optical waveguide, dispersion limits the length of the optical waveguide. Conversely, dispersion limits the pulse rate in an optical waveguide system having given optical waveguide properties and transmission distance.
Thus, there is a need in the art for optical waveguides and methods for fabricating them which reduce the amount of dispersion in an optical waveguide. Such dispersion-free optical waveguides would allow transmission of optical data signals over increased distances and at higher data rates relative to conventional optical waveguide systems which do not employ methods to reduce the dispersion in the optical waveguide. Moreover, there is a particular need in the art for an optical waveguide, and methods for fabricating them, having reduced dispersion and which do not require a predetermination of the length of the optical waveguide or its properties in order to properly compensate for the effects of dispersion.
The previously mentioned needs are satisfied by the present invention. The invention provides, in one embodiment, an optical waveguide that has zero or an otherwise inconsequentially small dispersion over a length of optical waveguide. In such a reduced dispersion optical waveguide, the length of the guide is not constrained by optical pulse spreading considerations. The present invention also includes methods for making such a reduced dispersion optical waveguide. The optical waveguide achieves such reduced dispersion by employing alternating regions of waveguide having positive and negative dispersion.
An optical waveguide composed of alternating regions having dispersion of opposite sign, has a small net dispersion. The dispersion in a particular region is compensated by the dispersion of opposite sign in the succeeding region. Thus, by fabricating an optical waveguide from a multiplicity of pairs of regions having such alternating sign of dispersion, the resulting optical waveguide has a net dispersion that is substantially reduced over a waveguide having a uniform dispersion, and which may be zero in an embodiment thereof. Moreover, it is a further advantage of the present invention that the reduced dispersion of the optical waveguide is obtained without having to first determine the overall length of the optical waveguide, or its dispersion properties.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.